| As for the limitations present in membrane distillation(MD) technology such as low membrane flux, membrane fouling, and bad stability of long-term operation, this paper introduced pulse flow, gas-liquid two-phase flow and a combination of both in the enhanced sweeping gas membrane distillation(SGMD) process with(Polyvinylidene fluoride, PVDF)hollow-fiber membrane to improve mass transfer and fouling control of membrane, leading to prolong the life of membrane effectively. The main achievements are as follows:(1) In the SGMD process enhanced by pulsed feeding(PSGMD), the effect of pulse parameters on membrane performance was explored, L16(45) OFF experiment was designed to find the primary and secondary factor of permeability performance, and a comparative study of pulse and steady feeding was made to examine permeate flux and membrane fouling.It was found that shorter pulse length(0.5 s) at a smaller pulse frequency of 1 min-1 could most efficiently increase permeate flux, up to 16.08 % improvement that of with steady flow.Moreover, L16(45) orthogonal fractional fractorial experiment results demonstrated that feed inlet temperature was the critical factor influencing the permeate flux and the best combination of parameter values were feed flowrate of 50 L·h-1, feed inlet temperature of 333 K, coolant temperature of 283 K, gas-sweeping flowrate of 0.84 m3·h-1 and filling factor of12.8%, respectively. Lastly, pulsatile feed flow with saturated NaCl aqueous solution(333 K)not only delayed the occurrence of sharp flux decline due to membrane pore blocking caused by crystal deposition as compared to continuous flow, but enhanced the thermal efficiency by more than 56.7%. Observations of fouling status on the membrane surface showed good agreement with the experimental results.(2) In the SGMD process enhanced by gas-liquid two-phase flow(GSGMD), the influence of bubble characteristics on membrane properties were researched. The results showed that longer bubbling interval(3 min) at a fixed bubbling duration of 30 s could most efficiently increase the the flux enhancement ratio up to 1.518. Next, the permeate flux increased with the gas flowrate under a relatively lower level, but tended to a plateau after the threshold level. Compared to non-bubbling case, the permeate flux reached up to 1.623 fold at a higher bubble relative humidity of 80 %. Furthermore, the better flux reinforcement effect could be achieved and meanwhile dramatic flux decline could be delayed for an intermittent bubbling system with a smaller nozzle size(2.2 mm). These results accorded well with the observations of fouling deposition in situ on the membrane surface with SEM.(3) In the SGMD process with enhancement of pulse flow & gas-liquid two-phase flow(P&G SGMD), the high-concentration saline(333 K saturated NaCl solution) as the feed, the permeability and hydrophobicity of membrane were tested in the P&G SGMD repeat test.Also, the effects of different reinforcement methods on permeate flux, membrane fouling,thermal efficiency were discussed. The repeat test indicated that the good reproducibility of the permeate flux and high hydrophobic property with retention rate of over 97.4%. As for strengthening contrast experiments, GSGMD > PSGMD > P&G SGMD in the aspect of flux enhancement. Also, the thermal efficiency were 29.40%, 30.74%, 33.05%, promoted by6.14%, 10.99%, 19.31% respectively. The SEM pictures of sedimentary layer on the membrane surface proved the permeate flux results.(4) The SGMD process was applied to seawater desalination, the effect of operation parameters on the membrane permeability, membrane stability of long-term operation, the impact of different optimization methods on permeate flux, membrane fouling, thermal efficiency were studied individually. When the feed flowrate and feed inlet temperature were fixed at 80 L·h-1 and 353 K respectively, the maximum permeate flux value of 2.0 L·m-2·h-1was got. During the 2180-min continuous trial run, the permeate flux remained relatively stable(~ 4.5 L·m-2·h-1), after it declined gradually until 2.41 L·m-2·h-1 at the end of the experiment. Results of strengthening contrast experiments indicated that the optimal flux enhancement effect could be obtained through bubbling method, its thermal efficiency was49.21%, increased by 27.76% relative to steady flow. The results of permeate flux were in accordance with the SEM pictures of membrane surface morphology. |
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